- Haskell-Cafe - Haskell.org

Appropriate location for list of language suggestions
by Will Yager 10 Aug '15

10 Aug '15

This email is in response to the (ongoing) Haskell-cafe discussion about type declaration syntax. There are a number of hypothetical changes to Haskell that could be aesthetically pleasing or convenient if implemented, but not to such a degree that the added convenience outweighs the added complexity of a pragma-gated feature or the cost of breaking existing code. (Case in point: renaming type/newtype/data to be less confusing to first-time users) However, there may be better opportunities to implement such changes in the future, and it would be a shame if every good (but perhaps somewhat trivial) idea was lost to the annals of the mailing list. Therefore, I proposed that we might maintain a list of nitpicks, which could be used to inform future decisions about breaking changes to the Haskell specification. I'm not sure what the most appropriate forum is for such a list. The mailing list is sub-optimal, because we might end up with a huge email chain littered with bikeshedding reply-alls. I would be willing to host and maintain such a list on Github, but that seems somewhat antithetical to the Haskell community's generally more distributed style of discussion. The wiki might be appropriate, but I don't know if it's conducive to proper moderation; we may have to filter out suggestions like "add for loops". --Will

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monads, memoization etc
by Ilya Razenshteyn 10 Aug '15

10 Aug '15

Hi everyone, I'm new to Haskell and decided to learn a bit about monads and their utility. Actually, what worked nicely for me was: to read first several pages from "Computational lambda-calculus and monads", then do exercises from https://tonymorris.github.io/blog/posts/20-intermediate-haskell-exercises/, and then map these exercises to actual functions in the standard library. Then, I decided to implement memoization to practice a bit with ST, HashTable's and such. Here is what I managed to get: http://pastebin.com/79pwjLPL . This code computes Fibonacci numbers while caching the intermediate values in a hash table. I tried to make the code as fast as possible (it runs in under 15 seconds, when computing 1M'th Fibonacci number modulo 999983). It uses the package hashtables. I have several question about all this. First, is there a cheap way to speed this code up? Note that I'm interested in a universal memoization strategy, that is I don't want to use arrays instead of hash tables etc. Second, is this code the right way of doing what it does (assuming that I really care about the performance)? Third question. Currently, the code passes "cache" everywhere. It would be natural to combine ST with something like ReaderT to store it there. What is a way to do it? I tried something like "ReaderT (HashTable s) (ST s) Int", but I have not managed to get it work.

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Re: [Haskell-cafe] Serialize to Haskell
by Corentin Dupont 08 Aug '15

08 Aug '15

Hi Matthew, yes that's exactly that. I want to do exactly what you would do with a JSON file (read, write), but with a data format that would be valid Haskell (instead of valid javascript). On Fri, Aug 7, 2015 at 8:06 PM, Matthew Pickering < matthewtpickering(a)gmail.com> wrote: > Hi Corentin, > > I don't quite understand your question please can you explain a bit > more. Do you want to read a valid haskell source file, perform some > changes to the file and then print out a valid source file? > > I am a bit confused about the bit about turing-completeness. > > Matt > > On Fri, Aug 7, 2015 at 7:55 PM, Corentin Dupont > <corentin.dupont(a)gmail.com> wrote: > > Hello! > > I'm wondering if it's possible to serialize some data to a format that is > > valid Haskell. > > In practice I would read a file like this: > > > > module Foo where > > > > myData :: MyData > > myData = MyData {name = "foo", > > descr = "test" > > ... > > } > > > > Reading this into a program is easily feasible with Hint, for example. > Then > > the program would modify some data and serialize it back to: > > > > module Foo where > > > > myData :: MyData > > myData = MyData {name = "bar", > > descr = "test2" > > ... > > } > > > > In practice I think that the format should be a subset of Haskell that is > > not Turing-complete. > > A bit like JSON, which is a subset of Javascript but not Turing complete. > > Is it possible? > > > > Thanks, > > Corentin > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > >

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Waiting for async exception (and nothing else)
by David Turner 08 Aug '15

08 Aug '15

Hi, Is there an elegant way to have a thread wait for an async exception but nothing else? I tend to do something like forever $ threadDelay 1000000000 but this seems like a bit of a hack. Waiting on an otherwise-unused MVar or doing 'atomically retry' don't work because GHC kills them off straight away. The context is when using an API that only has a 'withMyThingy' wrapper and no explicit openMyThingy and closeMyThingy alternatives. Perhaps that's an API problem and the only way to work around this is with something a bit hacky? Cheers,

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More concise code using phantom types
by Nikita Kartashov 08 Aug '15

08 Aug '15

Hello! Consider the following code: module Units where data Units a = U Double deriving Eq units :: Double -> a -> Units a units value _ = U value data Meters data Yards meters = undefined :: Meters yards = undefined :: Yards instance Show Meters where show _ = "meters" instance Show Yards where show _ = "yards" extractA :: Units a -> a extractA = undefined instance Show a => Show (Units a) where show u@(U value) = show value ++ " " ++ show $ extractA u main = (print $ units 5 yards) >> (print $ units 5 meters) Is it possible to use something instead extractA function here? For example, substitute "extractA u” with “undefined :: a”? GHC disallows it, so is there a way to explain that I only need a token with type a? Also, with highlighting on lpaste: http://lpaste.net/138219. With regards, Nikita Kartashov

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ANN: happstack-server-tls-cryptonite
by Andrey Sverdlichenko 07 Aug '15

07 Aug '15

Hi, I'd like to announce happstack-server-tls-cryptonite, package adding native TLS support to happstack, utilizing hs-tls and cryptonite packages by Vincent Hanquez. You can now run secure web server without using OpenSSL or other foreign libraries. This package is almost drop-in replacement for happstack-server-tls, just change Happstack.Server.SimpleHTTPS to Happstack.Server.SimpleTLS in imports. Regards, Andrey

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Fwd: Haskell Weekly News
by Kim-Ee Yeoh 07 Aug '15

07 Aug '15

*Editorial:*Over a month has elapsed since the last issue. Is the Weekly News turning into the Monthly News? No. I've been working on getting full-length articles into the News. Imagine articles that investigate, analyze, and give the low-down on hoary issues like cabal hell that cannot be telegrammed in a two-sentence paragraph. But my time isn't quantum mechanical. When I do A, I can't at the same time do B. I know that there are fine haskellers who'd gladly write for pay. However, HWN has never had a foundation in place for paid articles. Building that foundation takes time. Now that much of it is in place, let me duly announce that HWN hereby solicits reviews, reports, criticism pieces, and personal essays. - Flexible word count: anywhere between 250 and 750 is fine. - Send me an outline first. - Published articles pay at 68 bitcents, under 200 euros / dollars right now. - You can count on extensive editing for the requisite polish and also to conform to 'house' style. Interested? Email me with a subject line that starts with "[HWN pitch]" followed by the title of your piece. In the body, outline the main points you'll hit and explain how they engage the HWN readership. So on with the News : *Top picks:* - Galois organizes this year's ICFP contest. The 72-hour countdown has just started! After tantalizing us with whisperings of more computational archaelogy (see 2006 ICFP contest <http://www.boundvariable.org/>) centering on honey bees, national food security, and the Hebrew alphabet; Portland, Oregon reveals a classic offline AI game programming challenge <http://icfpcontest.org/spec.html>. According to redditor skatenerd <https://www.reddit.com/r/haskell/comments/3g1n8f/this_years_icfp_contest_st…>, "it's a lot like Tetris, except you know what pieces are coming." - Rick Dzekman <http://rickdzekman.com/thoughts/the-haskell-user-experience/> offers constructive criticism based on his "poor UX (User Experience) of a great language (Haskell)". Among the low-lying fruit he identifies: a web search doesn't always link to the latest version on the Hackage website, something easily fixed. Not so low-lying are Cabal, a modern IDE, and Haskell wiki documentation. In sympathy with the sentiments are HN readers <https://news.ycombinator.com/item?id=10008169> and /r/haskell <https://www.reddit.com/r/haskell/comments/3ftsh0/the_haskell_user_experienc…> . - Christoffer Stjernlöf brings attention to how only in imperative land does one encounter the awfulness of the same boolean expression getting tested twice in a nested if-inside-if. He uses it to make "the case for controlled side effects", the title of his blog post. Discussions on /r/haskell <http://www.reddit.com/r/haskell/comments/3fzmt6/the_case_for_controlled_sid…> and Hacker News <https://news.ycombinator.com/item?id=10017613>. - Alexey Shmalko introduces Haskell IO as "the command pattern" <http://www.alexeyshmalko.com/2015/io-is-your-command-pattern/> to object-oriented programmers. His article resists mentioning monads or do-notation. Appreciated on /r/haskell <http://www.reddit.com/r/haskell/comments/3fzmt6/the_case_for_controlled_sid…> . - The latest issue of the Functional Works monthly newsletter <http://us10.campaign-archive2.com/?u=30f19e2c5b9f15cfb95906716&id=cee1ab0dc6> reminds us about Chakravarty and Keller's "An Introduction to Computing (with Haskell)", a textbook written from multiple years teaching first-year CS. At 150 pages, the book dodges monads -- nary a single mention -- and zooms into I/O actions. It's out-of-print but readily downloadable <https://www.cse.unsw.edu.au/~chak/papers/#sec:books>. - The program for this year's Haskell Symposium <https://www.haskell.org/haskell-symposium/2015/> is out. PC chaired by Ben Lippmeier, it's truly a smorgasbord. If you can't find a preprint, indefatigable redditors <http://www.reddit.com/r/haskell/comments/3dxxj3/haskell_symposium_2015_acce…> might just have got you covered. - Zhenjiang Hu, John Hughes, and Meng Wang publish "How Functional Programming Mattered" <http://nsr.oxfordjournals.org/content/early/2015/07/13/nsr.nwv042>, a survey of FP's impact outside its academic domain in recent years. It is written in the mold of and complements A History of Haskell: Being Lazy with Class (2007) <http://research.microsoft.com/en-us/um/people/simonpj/papers/history-of-has…>. Comprising 17.5 pages of 2-column ACM-proceedings-style 9-point text, it houses the bibliographical treasure of 122 references, itself a contribution to any debate over priority. /r/haskell <https://www.reddit.com/r/haskell/comments/3f5yew/how_functional_programming…> - Victor Maia writes a lambda calculus evaluator in javascript <https://github.com/SrVictorMaia/optlam> that computes 200^200 mod 31 without breaking a sweat. No mean feat, since all numbers are church numerals. How did he pull it off? His javascript implements a fragment of Lamping's optimal algorithm, which is based on interaction nets. (Optimality here is in the sense of Levy-optimality: keeping the number of beta reductions down to an absolute minimum. By itself, this count is generally an inaccurate measure of efficiency.) He wonders on stackoverflow <http://stackoverflow.com/questions/31707614/why-are-%CE%BB-calculus-optimal…> why optimal evaluation has such magical superpowers. The convo at /r/haskell <http://www.reddit.com/r/haskell/comments/3f21wv/why_are_%CE%BBcalculus_opti…> has yet to crack the mystery. - Philippe Desjardins Proulx <http://phdp.github.io/posts/2015-04-05-automated-reasoning.html> codes and compares a basic arithmetic expression evaluator in all 5 languages of F#, Scala, Haskell, C++, and Julia. The evaluator acts on a given tree: Philippe doesn't touch parsing. Go, Erlang, and Elixir versions are in HN comments <https://news.ycombinator.com/item?id=9995896>. - Redditor gilmi starts a discussion on /r/haskell about the various FRP systems <https://www.reddit.com/r/haskell/comments/3fr5ij/frp_systems_discussion/> out there and whether they're ready for real-world use. Ollie Charles glosses classic vs arrowized systems, Doug Beardsley at Soostone testifies that "reflex is the first thing I've used that makes building web UIs enjoyable for me, which is very exciting", and Joseph Abrahamson channels Conal Elliott in staking out the acronym to mean exclusively those systems that admit continuous time. *Quotes of the Week:* - Andrew Cowie <https://twitter.com/afcowie/status/612764291863703552>: The highlight of my day is "cd ~/src/haskell/stack ; git pull ; stack install". What goodies have they landed this time? - Cale Gibbard <http://ircbrowse.net/browse/haskell?id=20873306&timestamp=1434453345#t14344…>: I basically never turn on IncoherentInstances because it basically means "I don't want anything to work, and I'd like to be confused about why." - Redditor fegu <https://www.reddit.com/r/haskell/comments/3fqiff/dont_give_up_on_haskell/ct…>: Haskell is like a mental drug, after a few initial hits you absolutely crave it. You will forego lucrative .Net career moves just to keep it, hence your long-term career will take a hit. But the days will be brighter, more enjoyable and there is just that glimmer of hope that in the end you will be able to practice Haskell full time in a well-paid position. I wish I was joking. - On ageism in the software industry <https://news.ycombinator.com/item?id=9710936#up_9711829>: Maybe I've been lucky, but this doesn't mesh with my experience at all. The oldest guys at my company are the ones teaching the classes about Haskell or Scalaz. They're the ones trying out Elixir. They're the ones building frameworks for everyone else to build on top of. The youngest people are banging out feature #4,501 for the website. *Real World Haskell of the Week:* - Fed up with the laggy Netflix UI? Check out the snappy Haskell-powered instantwatcher.com. H/T erichmond on HN <https://news.ycombinator.com/item?id=10013434#up_10016932>. -- Kim-Ee

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Serialize to Haskell
by Corentin Dupont 07 Aug '15

07 Aug '15

Hello! I'm wondering if it's possible to serialize some data to a format that is valid Haskell. In practice I would read a file like this: module Foo where myData :: MyData myData = MyData {name = "foo", descr = "test" ... } Reading this into a program is easily feasible with Hint, for example. Then the program would modify some data and serialize it back to: module Foo where myData :: MyData myData = MyData {name = "bar", descr = "test2" ... } In practice I think that the format should be a subset of Haskell that is not Turing-complete. A bit like JSON, which is a subset of Javascript but not Turing complete. Is it possible? Thanks, Corentin

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"cabal sdist" oddity
by Sven Panne 07 Aug '15

07 Aug '15

Cabal from HEAD started to issue the warning "Warning: Cannot run preprocessors. Run 'configure' command first." when running "cabal sdist" after a "cabal install", see e.g. https://travis-ci.org/haskell-opengl/OpenGL/jobs/74581138#L264. Previous versions didn't do this, see e.g. https://travis-ci.org/haskell-opengl/OpenGL/jobs/74581137#L264. Somehow the warning doesn't make sense, because the package has obviously been built before, so configuration must have happened, too. Is this a new bug in Cabal? Cheers, S.

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Syntax extension - adding import support to let/where bindings
by Oliver Charles 07 Aug '15

07 Aug '15

Hi all, I'm sure this has come up before, but a quick bit of Googling didn't reveal any prior discussions (if you known any, let me know), so I'm kicking off a new discussion. I find myself wanting to be able to say something like: foo = ... where import Something.Specific The result would be to import the contents of Something.Specific into the scope of foo and its other where bindings, but not import into the rest of the module that foo is defined in. As a motivating example, I'm currently working on building some HTML in Haskell, and the amount of symbols that come into scope is huge, when you have a DSL for both CSS and HTML - the real pain point being that you get symbols that often conflict. Here's an example of something that doesn't type-check currently image = img [ width 50, style [ width (px 50) ] ] width here is both a symbol in the HTML DSL and the CSS DSL, but to get this to type check I need to write something like image = img [ HTML.width 50, style [ CSS.width (CSS.px 50) ] ] That's not particularly bad here, the really pain is repeatedly having to do this for every single symbol I'm using. I'd prefer to be able to write image = let css = let import CSS in [ width (px 50) ] in let import HTML in img [ width 50, style css ] This is a little bit of a contrived rewrite, but hopefully it shows you what I'd like to be able to do. Please don't fixate too much on this example, it's only intended to illustrate the idea. When defining a large amount of inline styles I think this pays off - I import once to bring all the symbols I need into scope, rather than having to qualify every symbol. In reality, it will lead to me writing code like: myDocument = html where html = div [ style containerStyle ] [ div [ style rowStyle ] "Hello" , div [ style rowStyle ] "World!" ] where import HTML containerStyle = [ backgroundColor ..., padding ..., margin ... ] where import CSS rowStyle = [ backgroundColor ..., padding ..., margin ... ] where import CSS At present the suggestion is a syntax error, so I'm hoping that part won't be too controversial. Thoughts? *ocharles*

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